Wafer scale testing using a 2 signal jtag interface

ABSTRACT

Testing of die on wafer is achieved by; (1) providing a tester with the capability of externally communicating JTAG test signals using simultaneously bidirectional transceiver circuitry, (2) providing die on wafer with the capability of externally communicating JTAG test signals using simultaneously bidirectional transceiver circuity, and (3) providing a connectivity mechanism between the bidirectional transceiver circuitry&#39;s of the tester and a selected group or all of the die on wafer for communication of the JTAG signals.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of prior application Ser. No.13/175,484, filed Jul. 1, 2011, now U.S. Pat. No. 8,826,091, grantedSep. 2, 2014;

Which is a divisional of prior application Ser. No. 13/023,237, filedFeb. 8, 2011, now U.S. Pat. No. 8,015,463, granted Sep. 6, 2011;

Which is a divisional of prior application Ser. No. 12/849,191, filedAug. 3, 2010, now U.S. Pat. No. 7,904,774, granted Mar. 8, 2011;

Which is a divisional of prior application Ser. No. 12/493,926, filedJun. 29, 2009, now U.S. Pat. No. 7,797,602, granted Sep. 14, 2010;

Which is a divisional of prior application Ser. No. 12/175,655, filedJul. 18, 2008, now U.S. Pat. No. 7,571,365, granted Aug. 4, 2009;

Which is a divisional of prior application Ser. No. 11/565,979, filedDec. 1, 2006, now U.S. Pat. No. 7,417,450, granted Aug. 26, 2008;

Which claims priority from Provisional application Ser. No. 60/741,927,filed Dec. 2, 2005.

The following patent application publication includes subject matterwhich is related to the subject matter of this application. The patentapplication publication has been assigned to the assignee of thisapplication, and is incorporated herein by reference. US PatentApplication Publication, Pub. No. US 2006/0,236,174 A1.

The disclosure of this application also relates to U.S. Pat. No.6,073,254, “Selectively accessing test access ports in a multiple testaccess port environment.”

The disclosure of this application also relates to U.S. Pat. No.6,242,269, “Parallel scan distributors and collectors and process oftesting integrated circuits.”

The disclosure of this application also relates to U.S. Pat. No.6,543,020, “Test pattern compression for an integrated circuit testenvironment”.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The present disclosure relates generally to testing die on wafer and,more particularly, to testing of die on wafer using a reduced pin countJTAG interface. The reduced JTAG interface enables a larger number ofdie on wafer to be contacted by low cost testers and tested, whichresults in lowering the cost of manufacturing integrated circuits.

2. Description of Related Art

Today more and more wafer testing is being performed by low cost wafertesters. Low cost wafer testers are achieved primarily in two ways. (1)Decreasing the number of test contacts, and therefore expensive testerinterfacing circuitry and probing mechanisms, between the tester and dieon wafer, and (2) including more design for test circuitry (i.e. builtin self test circuitry, scan design circuitry, test datacompression/decompression circuitry) in the die to reduce the amount oftest circuitry required in the tester. Furthermore wafer test cost canbe reduced by using standardized design for test interfaces andcircuitry in the die, such as the IEEE 1149.1 (JTAG) interface. The IEEE1149.1 serial interface allows die on wafer to be tested via a standard5 wire interface consisting of a test data input (TDI), a test dataoutput (TDO), a test clock (TCK), a test mode select (TMS), and a testreset (TRST) signal. Using this interface, the tester only needs tocontact each die using these 5 test signals to input and output testdata and instructions. While the IEEE 1149.1 interface can be used forwafer testing, it still requires 5 contacts per die on wafer. Forexample, if 500 die on wafer are to be tested in parallel, the testerwould have to have 2500 contacts to the wafer (500 die×5 signals each).Additionally, the IEEE 1149.1 serial interface bandwidth is relativelylow since the TCK signal that times the interface typically operates ata frequency of only 20 to 50 MHz.

SUMMARY OF THE DISCLOSURE

The present disclosure provides a high speed, two signal test interfacebetween a tester and a selected group or all of the die on wafer. Thetest interface operates to communicate IEEE 1149.1 signals between thetester and die at high speeds through the use of simultaneouslybidirectional transceiver (SBT) technology.

DESCRIPTION OF THE VIEWS OF THE DISCLOSURE

FIG. 1A illustrates a first example of a tester contacting andcommunicating test data to die on wafer using the test interface of thepresent disclosure.

FIG. 1B illustrates a second example of a tester contacting andcommunicating test data to die on wafer using the test interface of thepresent disclosure.

FIG. 2A illustrates an example of the tester's test interface circuitryof FIG. 1A whereby one controller is used to control all the testerinterface channels.

FIG. 2B illustrates an example of the tester's test interface circuitryof FIG. 1B whereby each tester interface channel has its own controller.

FIG. 3 illustrates an example of a die's test interface channelinterfaced to IEEE 1149.1 TAP domains via a Tap Linking Module (TLM)according to the present disclosure.

FIGS. 4A and 4B, respectively, depict an example of the tester's testchannel circuitry and a diagram of the channel circuitry signals.

FIGS. 5A and 5B, respectively, depict an example of the tester's testchannel controller circuitry and a timing diagram of the controllersignals.

FIGS. 6A and 6B, respectively, depict an example of a die's test channelcircuitry and a timing diagram of the die's test channel signals.

FIGS. 7A and 7B, respectively, depict an example of the die's testchannel controller circuitry and a timing diagram of the controllercircuitry signals.

FIG. 8 illustrates an example of the interface between the SBT circuitryof the tester's test channel and the SBT circuitry of a die's testchannel.

FIGS. 9A-D depict case statements of the operation of the SBT circuitryof FIG. 8.

FIGS. 10A and 10B, respectively, depict an example of the 2-signalinterface between the tester's test channel and a die's test channel anda timing diagram of the interface signals.

FIG. 11 illustrates an example of performing scan testing using the dietest channel.

FIG. 12 illustrates an example of performing parallel scan testing usingthe die test channel.

FIG. 13 illustrates an example of performing test datadecompression/compression testing using the die test channel.

DETAILED DESCRIPTION OF THE DISCLOSURE

FIG. 1A illustrates an example of a tester 100 coupled to die 104-110 ona wafer 102 via a connectivity mechanism 124. The die 104-110 could be aselected group of die on the wafer or all of the die on wafer. Theconnectivity mechanism can be any type of connection scheme that allowssignals to flow between the tester and wafer. While not shown, thetester also supplies power and ground signals to the die on wafer topower up the die. The tester consists of a JTAG Pattern Engine 112 andtest channels 114-120. The JTAG Pattern engine consists of circuitrydesigned to communicate JTAG test patterns between the tester and wafer.The test channels receive the JTAG output test patterns (TMS and TDO)from the JTAG Pattern Engine and output them as serialized 2-bit packetson the Data I/O (DIO) paths 126-132 to each die on the wafer. The testchannels are simultaneously bidirectional which allows them tosimultaneously input JTAG test data inputs (TDI) from each die and inputthe test data input to the JTAG Pattern Engine, while, at the same time,the JTAG Pattern Engine outputs JTAG test patterns to the die. The testdata transferred between the tester and wafer via the DIOs 126-132 istimed by a clock signal (CLK) 134 which is provided by the tester.

Each die 104-110 of wafer 102 includes a test channel 136, a Tap LinkingModule (TLM) 1138, and TAP domains 140. The test channels 136 serve toreceive the serialized JTAG test data pattern (TMS and TDO packets) fromthe tester, via the DIOs, and convert the serialized JTAG test datapattern into a parallel JTAG test data pattern (TMS and TDI) which isinput to the TLM. The TLM is a circuit used to select one or more of theTAP domains 140 to be coupled for access by the tester. The TLMcircuitry can be implemented in a number of ways, including theimplementation described in U.S. Pat. No. 6,073,254 which isincorporated herein by reference. The test channels 136 aresimultaneously bidirectional which allows them to output JTAG test dataoutput (TDO) from a selected TAP domain(s) to the tester simultaneouslywith the input of the JTAG test data pattern (TMS and TDO) from thetester via the DIOs 126-132.

FIG. 1B illustrates another example of a tester 101 coupled to die104-110 of wafer 102 via a connectivity mechanism 125. The tester 101differs from the tester 100 of FIG. 1A in that each test channel 114-120is driven by a separate clock (CLK) from the JTAG Pattern Engine 113instead of a common clock (CLK) as shown in FIG. 1A. The advantages ofusing separate CLKs to drive the test channels include but are notlimited to; (1) the transfer of test data between the tester channelsand die channels can occur at the same or different frequencies, (2)different tests can be performed on each die, (3) each die test can bescheduled to occur at different times, and (4) each die test can bestarted, paused, or stopped at different times. All these advantages caneffectively be used to reduce the heat generated on the wafer during thetesting of a large number of die, which can be a major concern duringwafer testing.

FIG. 2A illustrates a more detail example of the tester 100 of FIG. 1A.As seen the JTAG Pattern Engine outputs JTAG TMS and TDO signals to eachtest channel 1-N, and inputs TDI data from each test channel 1-N. Acontroller 200 receives the CLK and TRST outputs from the JTAG PatternEngine, and outputs a load (LD) signal to the test channels and a CKINsignal to the JTAG Pattern Engine. The LD signal is used to cause thetest channels to load the parallel TMS and TDO data output from the JTAGPattern Engine which is shifted out of the test channel onto the DIO1-Nsignals. The CKIN signal output from the controller times the operationof the JTAG test pattern control circuitry within the JTAG PatternEngine. The TRST signal is used to initialize the controller 200. TheTRST signal is also input to the test channels for initializing them aswell.

FIG. 2B illustrates a more detail example of the tester 101 of FIG. 2B.As seen, each test channel 1-N has its own controller circuit 200 whichoperates in response to a unique CLK output from the JTAG Pattern Engine113 to generate the previously mentioned LD and CKIN signals. The JTAGPattern Engine of FIG. 2B has separate JTAG test pattern controlcircuitry which is interfaced to each of the test channels andcontroller combinations via separate TMS, TDO, CKIN, TRST, and CLKsignals. Each JTAG test pattern control circuitry in the JTAG PatternEngine is timed by the CKIN signal output from a channel controller 200.In response to the CKIN signal, each the JTAG test control circuitryoutputs TMS and TDO to the associated test channel and receives, duringJTAG DR or IR Shift operations, TDI input from the associated testchannel. The controller 200 of each test channel is timed by a separateCLK output 1-N from clock generation circuitry within the JTAG PatternEngine 113. As mentioned, the CLK 1-N outputs from the JTAG PatternEngine can be controlled to operate at different frequencies and/ortimes to adjust how each die coupled to a tester via the test channelsis tested.

FIG. 3 illustrates a more detail example of the die 104-110 of FIGS. 1Aand 1B. As seen, the die's test channel is coupled to the DIO and CLKsignals from a tester's test channel. The die test channel serves toconvert the serialized JTAG TMS and TDO patterns on the DIO signal backinto conventional parallel JTAG test TMS and TDO patterns and to applythe parallel JTAG TMS and TDI patterns to the TLM. The die test channelalso outputs the TCK and TRST signals to the TLM and receives the TDIinput from the TLM. The TLM serves to couple one or more of the die TAPdomains to the die test channel, via a conventional 5-signal JTAG bus,and then allows the tester to communicate with the coupled TAP domain(s)using the conventional 5-signal JTAG bus. As seen, the TAP domains maybe any type of TAP domain including but not limited to; Scan TAPDomains, BIST TAP Domains, Diagnostic TAP Domains, Debug TAP Domains,Fuse Programming TAP Domains, IEEE standard 1149.1, 1149.4, 1149.6, or1149.7 TAP Domains, IEEE standard 1500 TAP Domains, IEEE standard 1532TAP Domains, or any other type of domains using a TAP interface.

FIG. 4A illustrates a more detail example of the tester's test channel114 coupled to a controller 200. The test channel consists of a 2-bitparallel input serial output (PISO) register 402 and a simultaneouslybidirectional transceiver (SBT) 404. The TRST signal from the JTAGPattern Engine is used to initialize the controller 200 and testchannel's PISO 402. Following initialization, the PISO repeats the stepsof loading the TMS and TDO outputs from the JTAG Pattern Engine inresponse to the LD and CLK signals, then shifting out the TMS and TDOsignals serially on DIO in response to the CLK signal. The SBT allowsTDI input data from a die test channel, via the DIO signal, to be inputto the JTAG Pattern Engine while simultaneously the TMS and TDO datafrom the JTAG Pattern Engine is being output to the die test channel onDIO. The timing diagram, FIG. 4B, illustrates this simultaneous inputand output operation. The SBT is a known interface circuit that allowsdata to flow simultaneously in both directions over a single connection.A detail description of the operation of the circuit of FIG. 4 isprovided in regard to FIGS. 5A and 5B of patent application Pub. No. US2006/0,236,174. In FIGS. 5A and 5B of patent application Pub. No. US2006/0,236,174, the SBT circuit is referred to as input/output (I/O)circuit instead of SBT. The operation of the I/O circuit of Pub. No. US2006/0,236,174 and the SBT circuit of this disclosure is the same.

FIG. 5A illustrates an example design of the controller 200 whichconsists of two flip flops, three AND gates, and a Delay Circuitconnected as shown. The TRST signal resets the flip flops, the CLKsignal times the flip flops, and the gating and delay circuits decodethe states of the flip flops and the CLK input to produce the LD andCKIN output signals. As seen in the timing diagram, FIG. 5B, thecontroller responds to the CLK input to repetitively output the LD andCKIN signals. A detail description of the controller 200 is provided inregard to FIGS. 6A and 6B of referenced patent application Pub. No. US2006/0,236,174.

FIG. 6A illustrates a more detailed example of the die's test channel136 coupled to a TLM 138. The die test channel 136 consists of an SBTcircuit 602, a master reset and synchronization circuit (MRS) 604, a2-bit serial input parallel output (SIPO) circuit 606, a 2-bit register(REG) circuit 608, a TAP state monitor (TSM) circuit 610, a controllercircuit 612, and a power on reset (POR) circuit 614. The SBT circuit isused to input (IN) data from the DIO signal to the MRS and SIPOcircuits. The MRS circuit is used to reset the die channel circuitry,TLM, and TAP Domains, and to synchronize the operation of the diechannel circuitry with the tester's channel circuitry. The MRS outputs amaster reset (MRST) signal, which resets the channel circuitry, TLM, andTAP Domains, whenever the 5 TMS logic ones have been input on DIO andconsequently output on the TMS output of the REG.

Clocking, via TCK, the 5 TMS logic ones from the REG into the TSM, whichis a TAP state machine, causes the TSM to enter the test logic reset(TRST) state which sets the RST output of the TSM low, resetting the MRSand controller circuit. The TAP of the selected TAP domain(s) alsoenters the TRST state whenever the TSM enters the TRST state, whichresets the TAP domain(s). The reset state on the channel circuitry, TSM,and TAP Domains remains as long as DIO input logic ones on the IN inputof the MRS circuit. To transition from the reset state, asynchronization code from the tester, consisting of say 2 logic zerosfollowed by 2 logic ones, is input to the MRS circuit via DIO. Inresponse to the synchronization code, the MRS sets the CENA signal highfor a period of time to enable the operation of the controller to inputserialized TMS and TDO patterns into the SIPO to be output as TMS andTDI patterns to the TSM and TLM via the REG. With the controller andSIPO enabled, serialized JTAG TMS and TDI signal from the tester areagain output from the REG to cause the TSM to transition from the TRSTstate, which removes the logic low on the RST output of the TSM andenables the operation of the TLM and selected TAP Domain(s).

The SIPO circuit is used to input, via the IN signal, the serialized TMSand TDO data packets from the SBT circuit and output them in parallel tothe REG circuit. The REG circuit is used to hold the parallel TMS andTDI input signals to the TLM circuit stable as the next serialized TMSand TDO input packet is shifted into the SIPO circuit. The controllercircuit is used to update TMS and TDI data from the SIPO into the REG,via the update clock (UCK) signal, and to produce a test clock (TCK)signal which times the selected TAP domain(s) via the TLM circuit. TheTSM is used to allow the test channel circuitry to track the sixteen TAPstates of the connected TAP domain circuit. The POR circuit is used toforce the test channel, TLM, and TAP Domains into a reset state whenpower is first applied to the die. As seen in the timing diagram, FIG.6B, the die test channel inputs serialized TMS and TDO signals from theDIO signal and converts them into parallel TMS and TDI patterns, via theSIPO and REG, which are input to the TLM. Simultaneous with theinputting of the serialized TMS and TDO signals on DIO, the test channelcan output, during TAP IR or DR Shift operations, TDO data on DIO fromthe TAP Domain(s) selected by the TLM.

A detailed description of circuitry very similar to the test channelcircuitry of FIG. 6A is given in regard to FIGS. 7A, 7B, 8A, 8B, 9A, 9B,and 9C of referenced patent application Pub. No. US 2006/0,236,174. InFIGS. 7A and 7B of patent application Pub. No. US 2006/0,236,174, theSBT circuit is referred to as input/output (I/O) circuit instead of SBT.The operation of the I/O circuit of Pub. No. US 2006/0,236,174 and theSBT circuit of this disclosure is the same. In FIG. 6 of thisapplication the TLM is shown outside the TAP domain block, whereas inFIG. 7A of reference application Pub. No. US 2006/0,236,174 the TLM, ifused, is embedded within the TAP domain block and thus not shown.

FIG. 7A illustrates an example design of the controller 612 whichconsists of two flip flops and two AND gates connected as shown. Theflop flops can be reset by either the controller enable (CENA) or reset(RST) signals. The CLK signal times the flip flops and the gatingcircuits decode the states of the flip flops and the CLK input toproduce the update clock (UCK) and TCK output signals. As seen in thetiming diagram, FIG. 7B, the controller responds to the CLK input torepetitively output an update clock (UCK) to the REG and a TCK to theTLM and coupled TAP Domain(s). A detail description of the controller612 is provided in regard to FIGS. 8A and 8B of referenced patentapplication Pub. No. US 2006/0,236,174.

FIG. 8 illustrates the DIO connection between the SBT circuit of thetester's channel and SBT circuit of the die's channel. Forsimplification, the accompanying CLK signal connection is not shown. Thetester channel's SBT circuit consists of an output buffer 802, aresistor 803, and an input (I) circuit 804. The die channel's SBTcircuit includes an output buffer 806, a resistor 805, an input (I)circuit 808, and a pull up (PU) element 810. PU element 810 is used tohold the DIO input to the die channel high whenever the DIO input is notbeing externally driven, which forces the die's test channel, TLM, andTAP Domain circuits into a reset state, as previously described inregard to FIG. 6.

For simplification, the REG and the TLM circuits are not shown betweenthe SIPO and TAP Domains. The output buffers 802 and 806 serve to outputdata onto the DIO path from the tester and die channels respectively.The resistors 803 and 805 of the SBT circuits serve to limit currentflow when the tester and die channels are outputting opposite logicstates. The input (I) circuit compares the voltage level on the DIO pathto the logic voltage level being output from the tester and die channel.If the voltage level on DIO is the same as the logic voltage level beingoutput from the tester and die channel, the input (I) circuit inputsthat voltage level to the TDI input of the JTAG Pattern Engine of thetester channel and to the IN input of the SIPO of the die channel. Ifthe voltage level on DIO is at a mid-point level, indicating the outputbuffers 802 and 806 are outputting opposite logic voltage levels, theJTAG Pattern Engine will input the opposite logic level being outputfrom output buffer 802 and the SIPO will input the opposite logic levelbeing output from output buffer 806.

The simultaneous bidirectional data operation of the SBT circuits aredescribed in case statements A-D of FIG. 8 and illustrated in FIG. 9FIGS. 9A-9D. A detail description of the operation of the SBT circuitsis provided in regard to FIGS. 11A, 11B, 12, and 13 of referenced patentapplication Pub. No. US 2006/0,236,174.

FIG. 10A illustrates the DIO and CLK connections between a testerchannel and a die channel. For simplification, the die channel circuitonly shows the SBT 602, SIPO 606, REG 608, and Controller 612 circuitsand their associated signals, with the exception of the TRST signal.Also the TLM circuit 138 is not shown between the die channel andselected TAP Domain(s). It should be noted that in instances where thereis only one TAP domain in the die, the TLM circuit will not be requiredat all since that TAP domain will be permanently coupled to the diechannel as shown in FIG. 10.

The operation of the channels is shown in the timing diagram, FIG. 10B.As seen in the timing diagram, the operation includes repetitivelyserializing TMS and TDO output patterns from the JTAG Pattern Engineover the DIO path using the PISO 402, then de-serializing the TMS andTDO signals into parallel TMS and TDI inputs to the selected TAPDomain(s) using the SIPO 606 and REG 608. During the serialize andde-serialize operations, controller 200 outputs the LD signal to thePISO to load the TMS and TDO pattern from the JTAG Pattern Engine andoutputs the CKIN signal to time the operation of JTAG control circuitrywithin the JTAG Pattern Engine while, concurrently, controller 612outputs the UCK signal to update the REG with the parallel TMS and TDIsignals from the SIPO and outputs the TCK signal to time the operationof the TAP Domain. A more detail description of this serialize andde-serialize operation is provided in regard to FIGS. 14A, 14B, and 14Cof referenced patent application Pub. No. US 2006/0,236,174.

FIG. 11 illustrates a first example use of the present disclosure toperform scan testing of combinational logic within a core of an IC. Inthis example, a scan path is accessed by the die channel to shift instimulus test data, apply the stimulus data to combinational logic,capture the response of the combinational logic into the scan path, andshift the response data out of the scan path to the tester. The use of ascan path to test combinational logic is known. What is new is the useof the die channel to provide the stimulus test data input to the scanpath and the response test data output from the scan path.

FIG. 12 illustrates a second example use of the present disclosure toperform scan testing of combinational logic within a core of an IC. Inthis example, a SIPO register is used to serially input and applystimulus test data to inputs of parallel scan paths and a PISO registeris used to capture and shift out response test data from the outputs ofthe parallel scan paths. The use of a SIPO and a PISO to input andoutput test data to parallel scan paths is known, see U.S. Pat. No.6,242,269. What is new is the use of the die channel to provide thestimulus test data input to the SIPO and the response test data outputfrom PISO.

FIG. 13 illustrates a third example use of the present disclosure toperform scan testing of combinational logic within a core of an IC usingdecompression and compression circuits. In this example, a decompressercircuit is loaded with compressed stimulus test data patterns from TDIwhich are decompressed by the decompresser and input to the parallelscan paths, while the compresser circuit compresses the output responsesof the parallel scan paths and outputs the compressed response test datato the tester. The use of test data compression and decompressioncircuits is known, see U.S. Pat. No. 6,543,020. What is new is the useof the die channel to provide the compressed stimulus data input to thedecompresser and the compressed response data output from thecompresser.

Although exemplary embodiments of the present disclosure are describedabove, this does not limit the scope of the present disclosure, whichcan be practiced in a variety of embodiments. Further, references todetailed descriptions in referenced patent Pub. No. US 2006/0,236,174are intended to provide more detailed teaching of the operation ofcircuits similar to those described in this application. It should beunderstood that subtle differences in circuit and signal names may beseen between the circuitry described in Pub. No. US 2006/0,236,174 andthe circuitry contained in this application.

What is claimed is:
 1. An integrated circuit wafer comprising: (a) afirst data input and output lead, a second data input and output leadseparate from the first data input and output lead, and a wafer clocklead; (b) a first die including: (i) first channel circuitry having: (A)a channel interface that includes a bidirectional lead connected to thefirst data input and output lead for carrying a test mode select signal,a test data in signal, and a test data out signal, and including achannel clock lead connected to the wafer clock lead, and (B) a linkingmodule interface that includes a test mode select output lead, a testdata in output lead, a test data out input lead, and a test clock outputlead; (ii) first TAP domains, each first TAP domain being connected tofirst functional logic and having a TAP interface that includes a testmode select input, a test data input, a test clock input, and a testdata output; and (iii) first TAP linking module circuitry having a firstinterface connected to the linking module interface of the first channelcircuitry and second interfaces, each second interface being connectedto the TAP interface of a first TAP domain; and (b) a second dieincluding: (i) second channel circuitry having: (A) a channel interfacethat includes a bidirectional lead connected to the second data inputand output lead for carrying a test mode select signal, a test data insignal, and a test data out signal, and including a channel clock leadconnected to the wafer clock lead, and (B) a linking module interfacethat includes a test mode select output lead, a test data in outputlead, a test data out input lead, and a test clock output lead; (ii)second TAP domains, each second TAP domain being connected to secondfunctional logic and having a TAP interface that includes a test modeselect input, a test data input, a test clock input, and a test dataoutput; and (iii) second TAP linking module circuitry having a firstinterface connected to the linking module interface of the secondchannel circuitry and second interfaces, each second interface beingconnected to the TAP interface of a second TAP domain.